A vehicle navigation system is taken as an example of image operation systems. A user can scroll the map displayed in a display or move a pointer or a button selection cursor in a menu screen by using a remote controller (pointing device) with an input operation section such as a joystick when using this type of vehicle navigation system. When a map scrolls on the display, the map is moved by a predetermined distance in the opposite direction on the instruction signal sent from the remote controller. When selecting a button with a button cursor, the cursor is moved by one button in the signaled direction. When the same signal is sent over a predetermined period of time, the map is scrolled in the same direction successively or the button cursor is moved to the next button successively.
That is, in the vehicle navigation system, a desired operation in a screen image can be achieved when the pointing device such as a remote controller communicates to the vehicle navigation system uni-directionally to send an operational status signal of the pointing device.
On the other hand, for the purpose of operability, a system that has a reactive force on its input operation section according to the display position of a pointer or the like by using a bilateral communication between the pointing device and the vehicle navigation system is becoming close to practical use. That is, a reactive force is applied to the input operation section of the pointing device for the ease of placing the pointer on top of a selection button and the like in the screen image. When the reactive force is introduced, objects in the screen image act according to the Newton's third law of motion, simulating a real world of gravity. For example, the reactive force, when decreasing and then increasing along an X-axis, is integrated as a ‘potential’ of a drawing force on the pointing device. As a result, the pointing device can comfortably be guided to the bottom of the ‘potential’ curve, even when arbitrarily operated by an operator. According to this method, the operator can easily place the pointer on top of the area of selection button in the screen image, once the minimum reactive force is placed in the target area.
However, when a bilateral communication like this system is used, the following problem of screen image replacement arises. That is, a reactive force data is formed on a screen image basis to be applied to the input operation section. This reactive force data is outputted from the navigation system to the pointing device according to the screen image to be displayed on a screen. On the other hand, when an image data to generate a screen image is formed in the navigation system and is outputted to the display device, generation of a screen image takes time and a period of generation time varies according to the amount of image data. Therefore, the reactive force being applied to the input operation section and the screen image being displayed does not match and may cause a sense of discomfort to an operator.